In numerous situations, it is useful to be able to transmit information to a person other than by acoustic or visual means. This is the case particularly when one wishes to discreetly alert a person who is in the middle of a group of people. Tactile means for transmitting the information thus offer an advantageous alternative: a unit that the person is carrying close to the body, such as a watch, for example, is made to vibrate, in order to stimulate his skin locally to indicate to him a given time or the occurrence of an event (arrival of a message, a call, a meeting etc.). Such tactile information transmission means find application in a device for indicating to people, whose keenness of sight is reduced or non-existent, the time, the occurrence of an alarm or any other event. By way of information, reference can be made to European Patent Application Nos. EP 0 710 899 and EP 0 884 663, both also in the name of the Applicant, which disclose timepieces incorporating a vibrating device.
Unbalance type vibrating devices mounted on a rotor are known to those skilled in the art. In these devices, typically, the unbalance rotates at a speed of several tens of revolutions per second thanks to an electric motor powered at a power of several tens of milliwatts and started at the moment when the occurrence of an event has to be perceived by the wearer.
These devices have the main drawback of consuming a lot of energy, which is incompatible with the requirement to miniaturise batteries and components encountered in the horological field.
European Patent Application No. EP 0 625 738 in the name of the Applicant discloses a device for making a unit such as a watch vibrate. This device includes a coil electromagnetically coupled to a moving mass.
This Patent Application does not disclose the features of the coil excitation means. Having said this, those skilled in the art know that pulses whose frequency is equal to the natural mechanical oscillation frequency of the mass have to be applied to the coil in order to obtain maximum vibration amplitude for a given quantity of supplied energy.
However, in practice, this natural frequency is difficult to determine rigorously. First of all, it varies from one moving mass to another because of manufacturing tolerances, which are of the order of 15%. Then, it varies as a function of the way in which the coil-moving mass unit is carried, and the extent to which it is worn close to or remote from with the wearer's body. Typically, the carrying conditions induce variations of the order of 5% in the natural frequency of the unit, as well as a variation in the dissipated energy. These variations decrease the yield of the excitation means that are designed to operate at a fixed frequency, and this results in a significant waste of energy.
It is a general object of the present invention to overcome these drawbacks.
It will be noted that those skilled in the art already know, from U.S. Pat. No. 5,436,622, a vibrating device including a coil-moving mass unit which is activated, during a first phase, at a frequency substantially equal to a nominal natural oscillation frequency of the moving mass, then, during a second phase, is left in free oscillation in order to determine the natural oscillation frequency of the unit, which depends on the conditions in which the device is worn by the user. Once the natural oscillation frequency has been determined, the moving mass is driven at this frequency for the entire remaining duration of the vibration.
According to this document, it will be noted that the vibrating device is made to vibrate by a periodic rectangular signal of equal frequency to the determined natural frequency, for the entire period that the moving mass is made to vibrate. This appears clearly, for example, in FIG. 3 of U.S. Pat. No. 5,436,622. According to this document, the vibrating device is thus continuously driven and is never left in free oscillation during the period that the device vibrates.
Given that the natural oscillation frequency of the unit is dependent on the conditions of wear, this frequency can vary substantially during the period that the device vibrates. Thus, a major drawback of the device disclosed in the aforementioned U.S. Pat. No. 5,436,622, lies in the fact that it cannot respond to a modification in the natural oscillation frequency during vibration of the vibrating device, the measurement only being carried out when the device is next activated. The energetic yield of the device is thus not optimal, such that an alternative solution has to be sought. According to this U.S. Pat. No. 5,436,622, it is suggested in particular that the vibrating device be fitted with an additional sensor for measuring the oscillation frequency, as this appears in FIG. 5 of this document, in order to allow the oscillation frequency of the vibrating device to be adapted during the oscillation in progress.
European Patent Application No. EP 0 938 034 in the name of the Applicant discloses an advantageous solution according to which the natural oscillation frequency of the vibrating device is determined during each period (or half-period) of oscillation of the moving mass. Unlike the solution disclosed in the aforementioned U.S. Patent, this solution thus allows the variations in the natural resonating frequency to be taken into account when the device is made to vibrate, without it being necessary to use an additional sensor. Here, the device is driven in vibration, not by a periodic rectangular signal of determined frequency, but by a succession of positive and negative pulses generated during each half-period of oscillation at the end of time intervals that are a function of the instantaneous oscillation frequency of the moving mass measured during the preceding period. Between the driving pulses, the device oscillates freely such that measurement of the instantaneous natural frequency is possible.
The Applicant was able to observe that this solution could have a drawback in certain conditions. Without adequate control means, this solution can, in particular, be subjected to measuring errors which would result in driving the vibrator at an inadequate frequency. Indeed, in the event that a measuring error occurs, this measuring error is then repeated during the following oscillations, such that the device quickly becomes unstable. In order to avoid this risk, the device then has to be designed such that this instability is prevented.
One solution to this problem may consist in alternating the periods during which the natural oscillation frequency is measured and the periods during which oscillation of the vibrating device is maintained in order to let the latter vibrate freely and allow reliable measurement of the natural oscillation frequency. This solution is not, however, appropriate because of the rapid damping of the oscillations, which involves generating a driving pulse of greater intensity in order to maintain the oscillation of the unit and which consequently generates higher power consumption.
It is thus another object of the present invention to propose an alternative solution to that disclosed particularly in European Patent document No. EP 0 938 034 which allows an adequate response to be made to variations in the natural oscillation frequency of the device and which remains easy to implement.
It is also an object of the present invention to propose a solution that is more robust and more stable than the solutions of the prior art.